python: Drop unicode type.

python: Drop unicode type.

Python 2 had str and unicode.  Python 3 only has str, which is always a
unicode string.  Drop use of unicode with the help of six.text_type
(unicode in py2 and str in py3) and six.string_types ([str, unicode] in
py2 and [str] in py3).

Signed-off-by: Russell Bryant <russell@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>